The present invention relates to a grab for excavators of foundation walls and rectangular piles, which includes a frame and two substantially symmetrical jaws articulated onto the frame, which jaws can open and close with respect to each other.
Several types of foundation wall excavators are known, being used for digging trenches for laying the foundations of walls and the like.
In particular, classic machines have a grab which includes a frame and two symmetrical jaws, articulated onto the frame, which can open and close with respect to each other thanks to two hydraulic cylinders. The grab unit is mounted on a vehicle and can either be suspended from a cable or mounted on the lower end of a support bar, usually square, which can slide vertically and is known as a xe2x80x9ckellyxe2x80x9d.
The grab is made to descend open until it rests on the ground to be dug; it is then closed in such a way as to pick up the earth which is trapped between the two jaws and remove it from the site.
All these known machines share one common disadvantage: the force that the grab exerts on the site when it closes, that is, the extraction force, is relatively small, especially so in the case of grabs suspended from a cable, because that force is based exclusively on the weight of the grab.
In the case of grabs mounted on a kelly a greater force can be exercised than with suspended grabs, especially if the kelly is braked, because the reaction is transmitted to the vehicle; it nevertheless remains a relatively small force, and in some cases the reactions on the kelly even lift the vehicle, with the consequent danger involved.
Machines with a kelly can exert more force, but they have disadvantages when compared with cable machines, such as their greater cost and a limitation of the digging depth to the length of the kelly itself, which can be made up a limited number of telescopic sections.
In order to make greater digging force available for cable-suspended grabs, there has been a tendency in recent years towards increasingly heavy grabs. This solution nevertheless involves many disadvantages, such as the fact that the vehicle must also be heavier and more robust, with the consequent increase of cost and volume. Excavators that have achieved greater power based on weight are only manageable in open zones and cannot work in places where access is more restricted.
One recently proposed alternative to machines with grabs consists in an excavator with a milling head which reduces the rock to powder, and a water-pump extraction system. Such machines do satisfactorily resolve the problem of digging force on solid rock, but they have other disadvantages which restrict their use considerably: on the one hand, their they are much larger and more costly than machines with grabs, and, on the other hand, they are unsuited to most terrains, because any loose stones on the site are not broken up, they elude the milling mechanism and clog the extraction systems.
The objective of this invention is to solve the aforesaid disadvantages by developing a grab of the type mentioned in the introduction that is capable of exerting greater force against the terrain but can at the same time be lighter and of relatively low cost.
In accordance with this objective, the grab of the invention for foundation wall and rectangular pile excavators is characterised in that it includes at least two elements of support against the walls of the hole that the grab is digging and means for moving those supporting elements between a folded-in position in which they remain alongside the frame and an unfolded position in which they are pressed against the walls of the hole.
Thanks to the presence of these supporting means the grab can dig exerting much more force than if it relied solely on its own weight, and this advantage is achieved without significantly increasing either the volume or the weight of the machine or the grab. Additionally, the grab of the invention provides these advantages whether it is associated with a singe-section or telescopic kelly or is suspended from a cable.
Another advantage of the supporting means is that they help to keep the grab firmly in the hole, preventing it from deviating, due for example to any slight asymmetry between the jaws or to unevenness of the terrain.
Preferably, the means for moving the supporting elements between the folded-in position and the unfolded position actuate automatically when the grab reaches the bottom of the hole during the downward movement of the grab.
The supporting means thus come into action in synchronisation with operation of the grab, and it is not necessary to operate them manually each time the grab is lowered.
In accordance with the preferred embodiment of the invention, the grab includes at least one kelly section, to which the frame with the jaws is attached in such a way that it can move vertically between an upper stop and a lower stop, so that when the grab is not resting on the ground the frame is suspended from the kelly, and when the grab is resting on the ground the kelly rests on the frame.
This embodiment is mechanically simple, and allows the movement between the kelly and the frame to be used to drive the supporting means mechanically.
In this case, the means for moving the supporting means advantageously include a kinematic chain that links the kelly, the frame and the supporting elements, in such a way that when the jaws and the frame reach the bottom of the hole the kelly descends until it rests on the frame, and during this descending movement the kinematic chain causes travel of the supporting elements until they rest pressing against the walls of the hole.
This solution is robust and not very costly, and allows the automated operation mentioned above.
In one embodiment, said kinematic chain includes two arms, each articulated at one end to the kelly and at the other end to at least one link which is, in turn, articulated to the frame and to a supporting element. Each supporting element can include a long shoe with a vertical surface that comes into contact with the wall of the hole, while the kinematic chain can further include, for each shoe, an articulated parallelogram mounted between the frame and the shoe, so that the latter is kept substantially vertical during its displacement.
Preferably, the grab includes means for adjusting the maximum displacement between the kelly and the frame.
This characteristic allows the pressure of the supporting means against the walls of the hole to be adjusted to adapt it to the different types of terrain and different situations.
Also preferably, the grab includes means for locking the kelly and the frame to each other so that the mechanism can be overridden in situations in which it might prove to be a hindrance.
In one embodiment, the supporting elements have a smooth contact surface with the walls of the hole; alternatively, the supporting elements can have a contact surface with the walls of the hole which is provided with rubber projections or teeth. This last option is useful to give greater adherence against the walls or to cause milling of the hole wall, which can be desirable if, for example, one of the walls of the hole is concrete belonging to an adjacent section of the foundation wall.
In another aspect, the invention relates to an excavator for foundation walls and rectangular piles, characterised in that it is provided with a grab with the described features.
In one embodiment of the machine, the grab is mounted on one end of a kelly which can travel vertically on a corresponding support of the machine; the kelly can further be telescopic.
This embodiment has the usual advantages of the machines having a kelly, that is, greater control of the position of the grab and greater digging force, due to transmission of stresses to the machine, although in the case of the invention most of the force is borne by the supporting elements.
Alternatively, the grab can be mounted on the end of a kelly section which is in turn suspended from a cable attached to the machine. In this case, the machine still has all the advantages of the new grab of the invention in terms of digging force, and the perforation depth can be greater.